For some applications, for example for uses in the petrochemical industry, there is a requirement that a rotary valve can in the event of a fire still be closed to prevent flow along a pipeline. In rotary valves it is known to use seals of materials, such as materials based upon polytetrafluoroethylene (P.T.F.E.), which may be destroyed in the event of a serious fire, and provision must accordingly be made for a fireproof back-up seal which would survive a fire of the expected intensity.
Such valves are known as `fire-safe valves` and meet well known standards such as British Standard No. 5146.
There has been proposed in U.K. patent specification No. 1 264 686 a butterfly valve in which the valve disk is mounted for pivotal movement on a shaft of which opposite end portions are rotatably mounted in fire-destructible bearings in the valve housing. The valve comprises an annular seal, which seals between the periphery of the disk and the wall of the passageway when the valve disk is in a closed position, comprising a deformable sealing element of P.T.F.E. and a rigid fireproof ring. There is a clearance between the shaft on which the disk is mounted and the valve housing so that in the event of a fire, which destroys both the sealing element and the bearings supporting the shaft in the housing, the shaft is free to move radially for movement of the disk along the passageway into sealing engagement with the fireproof ring. The disk is moved into engagement with the ring by fluid pressure within the passageway.
One end portion of the shaft is mounted in a blind bore in the housing, but the opposite end portion extends out of the housing to provide means for actuation of the valve. In order that fluid should not escape from the passageway through the clearance between said opposite end portion of the shaft and the housing when the bearing sleeves are destroyed, a deformable, fireproof, seal is provided in the clearance. However, this seal has the adverse effect of restricting the radial movement of the shaft in the clearance, and will tend to cause a pivotal movement of the shaft about the seal. As a consequence of such pivotal movement, the valve disk may not make adequate sealing engagement with the fireproof ring and the valve may not be fully closed. Furthermore, owing to the distortion of the seal around the shaft, when the shaft moves upon destruction of the bearings, the seal may no longer be fully effective and may allow the escape of fluid from the passageway.